Chemical reactions cannot occur until individual molecules of the reagents are brought together, and physical interactions between components are greatly facilitated as the components are more and more intimately mixed together. Bulk stirring is only able to present the opportunity for reagent molecules to contact one another after sufficient time has elapsed to provide the necessary uniformity of interdispersion of the reagents' molecules for achieving the desired one on one contact which finally makes a reaction possible, and only molecular diffusion can accomplish the required one on one contact, which is a very slow process. These encounters can be helped to occur by establishing small scale fluid structures or eddies within which molecular diffusion becomes significant. The role of the reactor, and the mixing and mass transfer equipment associated with it, is to create these small scale fluid structures in order to generate and improve mixing, mass transfer and molecular inter-diffusion. The reactor equipment must therefore direct energy into the fluid system in the correct way. In a stirred tank reactor (STR) the energy input clearly comes from the impeller, but this arrangement suffers from high energy losses through friction, macro-agitation, mere recirculation of the fluid, and other factors. The energy which is usefully employed is focused mainly upon the fluid in contact with the impeller, particularly with its leading edges, along which occurs the only action which can be called forced, molecular inter-diffusion. This means that while the power input at the impellor tip may be very high (e.g. 1000 W/kg) the majority of the fluid is not undergoing forced molecular inter-diffusion, and the average power input across the whole tank producing conversion is low (e.g. 0.1–1 W/kg).
A further important disadvantage of bulk agitated chemical reaction systems is the fact that dimensional scaling up or down also changes the kind and quality of the resultant product. Very often, time consuming trial and error experimentation is required after a change in vessel dimensions. It may take as many as 5 years for some reactions to be scaled up from test tube to a fully undustrial sized apparatus. This handicap is a consequence of the changing ratio of wet volume to wetted surface areas when dimensional changes of the apparatus are made which will change the corresponding hydraulic radius and in turn the resulting Reynolds number of the agitated fluid. The larger the ratio of wet volume to wetted surface becomes the more difficult the scaling up. For this reason, chemical engineers have been trying to move into the other direction and by raising the wetted surface to wet volume ratio and compensate the lost economy of large scale by improving the intensity of bulk agitation.
The typical advances that have been obtained in improved mass transfer are by use of what is known as high-power, rotor-stator mixers, where the proportion of the fluid volume in contact with the rotor surface is much lower, and by use of static mixers and ejectors where the large amount of energy which can be supplied by pumps goes into the whole of the fluid hold-up volume through intensified supra-Kolmogoroff agitation. In this way higher power inputs (e.g. 100W/kg) can be created, followed by improved mass transfer. However, such apparatus suffer from the inability to effect continuous, high-speed, uniform and forced inter-diffusion of reactant molecules on a sub-micron and nanometer scale in addition to the inadequate thermal control available, for example, with highly exotherm, fast reactions. Another type of apparatus that has been employed comprises static micro mixers, which can produce mixtures of liquids and gases, as well as generate multiphase dispersions. Such devices, which can be manufactured using methods borrowed from the electronics industry, consist of a series of very small channels engraved or etched, for example, into a silicon wafer surface, through which the reaction components are passed together in laminar flow mode; the channels can for example be as small as 10 micrometers in diameter. The mixing mechanism is based on flow multilamination with subsequent interdiffusion of molecules between the overlapping fluid lamellae. When used as a reactor the reduction of the diffusional path length results in accelerated mass and heat transfer. Despite the improved mass transfer obtainable with the above mentioned equipment, many reactions are very slow because they are still diffusion controlled and therefore their rate depends on slow, natural, unforced, molecular inter-diffusion.
There is therefore increasing interest in what has been referred to as process intensification technology, fueled primarily by the need to provide industrial processes that are more efficient and economical than those employed to date. Such technology is applied to any physical and/or chemical process involving heat and/or mass transfer and/or physical and/or chemical reaction, the latter term including both chemical composition and decomposition, and it generally involves producing on, and/or introducing to, a moving surface a thin film or its equivalent (see below) of each of the process components, so that interaction between them is greatly facilitated. It is also found that such interactions are possible under conditions of temperature and/or pressure that can be relatively closely controlled. When a process component has the form of a gas, or a vapor, or a plasma, it may be introduced to the surface in a form which is equivalent to a thin film, for example by bathing the surface in the component, or as a flow of the required thin dimension.
One way in which process intensification technology has been implemented is known as Spinning Disc technology, in which a body providing a disc-like surface, which may be flat or conical, is rotated about a spin axis to create centrifugal force across the surface. The process components are introduced on to the disc surface at or adjacent to the spin axis, whereby the component(s) flow radially outward under the centrifugal force in the form of a thin film. Such apparatus was proposed initially for typical heat and mass transfer operations, and subsequently has been adapted for use as a reacting surface. The employment of the process component(s) in the form of very thin films also facilitates the application to the material(s) of different types of energy that will assist in promoting the process intensification, such as electromagnetic radiation or longitudinal pressure oscillations. Examples of such spinning disc apparatus, and their methods of operation, are described in U.S. Pat. No. 4,549,998 and PCT applications Nos. PCT/GB00/00519; PCT/GB00/00521; PCT/GB00/00523 and PCT/GB01/00634, all in the names of Colin RAMSHAW et al.
Professor Colin Ramshaw and others of the Process Intensification and Innovation Centre (PIIC) at Newcastle University, England have developed processes and apparatus for continuous production of nano particles from various reactions using thin, highly sheared films on the top surface of a single rotating disk, usually now referred to as a Spinning Disc Reactor (SDR). Unsteady film surface waves on the disc surface, coupled with the shearing action of the rotating surface, ensure that micro mixing is achieved. These films are less than 100 microns thick and so offer a short diffusion path length, resulting in excellent heat and mass transfer. Residence times on the SDR range from a few seconds down to fractions of a second, and it is therefore well suited to fast processes where the inherent reaction kinetics are of the same order or faster than the mixing kinetics.
An evaluation of spinning disk reactor technology for the manufacture of pharmaceuticals was published in Industrial & Engineering Chemistry Research 2000, Vol 39, Issue 7, pp 2175–2182 by Brechtelsbauer C.; Ricard F.; Lewis N.; Oxley P.; and Ramshaw C. A continuously operating SDR displayed distinct advantages over batch processing techniques when several processes for the manufacture of pharmaceuticals were investigated as test reactions. It proved to be a useful tool for revealing intrinsically fast kinetics as well as for optimizing processes with such kinetics. Very encouraging results were achieved for a phase-transfer-catalyzed (ptc) Darzen's reaction to prepare a drug intermediate and the recrystallization of an active pharmaceutical ingredient (API). In comparison to presently used batch processes the ptc reaction with the SDR had a 99.9% reduced reaction time, 99% reduced inventory, and 93% reduced impurity level. The recrystallization yielded particles with a tight particle size distribution and a mean size of around 3 μm.
An evaluation of an SDR for continuous processing was published in Organic Process Research & Development 2001, Vol 5, Issue 1, pp 65–68, again by Brechtelsbauer C.; Ricard F.; Lewis N.; Oxley P.; and Ramshaw C. The results obtained for two organic reactions and one crystallization are discussed. The SDR was found to be a useful tool for revealing intrinsically fast kinetics as well as for optimizing a process with such kinetics. Control of particle size distribution was demonstrated with the crystallization investigated.
An evaluation of the use of an SDR in the application of electromagnetic radiation to chemical processes was given in a paper entitled Photo-initiated Polymerization Using A Spinning Disc Reactor by Dalglish, R.; Jachuck, A and Ramshaw, C. of the Process Intensification & Innovation Centre (PIIC), Newcastle University, England, presented at a conference entitled Process Intensification in the Chemical Industry, Antwerp, Netherlands, 25th Oct., 1999. The results of photo initiated polymerization studies carried out at PIIC using a spinning disc reactor are discussed. Initial results have been promising and suggest a novel route for fast, controlled and continuous polymerization of free radicals. The effect of UV intensity, film thickness of the monomer/polymer film and the rotational speed in the rate of polymerization has been studied. It is hoped that this technique may be used to perform polymerization reactions in seconds rather than hours.